Detergents & Surfactants

Surfactants constitute the most important group of detergent components. Generally, these are water-soluble surface-active agents comprised of a hydrophobic portion, usually a long alkyl chain, attached to hydrophilic or water solubility enhancing functional groups.

A list of BioUltra surfactants which are commonly used in biochemistry is presented in the table below.

Structures of common surfactants used in biochemistry:

Sodium cholate

Sodium dodecylsulfate (SDS)

Sodium deoxycholate (DOC)

N-Lauroylsarcosine Sodium salt

Lauryldimethylamine-oxide (LDAO)

Cetyltrimethylammoniumbromide (CTAB)

Bis(2-ethylhexyl)sulfosuccinate Sodium salt

Fig. 1: Typical decrease of the surface tension of water by surfactants.

All surfactants possess the common property of lowering surface tension when added to water in small amounts, as illustrated in Fig. 1.The characteristic discontinuity in the plots of surface tension against surfactant concentration can be experimentally determined. The corresponding surfactant concentration at this discontinuity corresponds to the critical micelle concentration (CMC, see Fig. 1). At surfactant concentrations below the CMC, the surfactant molecules are loosely integrated into the water structure (monomer see Fig. 2). In the region of the CMC, the surfactant-water structure is changed in such a way that the surfactant molecules begin to build up their own structures (micelles in the interior and monolayers at the surface, Fig. 2).

Fig. 2: Equilibria between the surfactant monomer, surface monolayers and micelle in an aqueous medium

Due to the limited solubility of surfactants in water, aggregates are formed in which the hydrophobic or hydrophilic sections of the surfactant are stuck together [1, 2]. The micelle may be represented as a globular cylindrical orellipsoidal cluster [3] of individual surfactant molecules in equilibrium with its monomers [2-14]. The reverse orientation of the hydrophilic and hydrophobic portion of the surfactant in a hydrocarbon medium leads to reversed micelles [15]. Planar bilayers or small unilamellar vesicles are also formed [16]. The extraordinary variety of the phase behavior of surfactants in solution can further be broadened by the inclusion of additives or co-surfactants [17-19].

The micelle formation process can be described by the following equation:

m * S <=> Sm

where m is the average association number S the concentration of monomeric surfactant and Sm

The most complete listing of critical micelle concentrations can be found in the United States Department of Commerce Publication [20]. Additional data from the relevant literature have been included in the product description.

One class of additives are proteins. Their interaction with surfactant systems has received increasing attention in recent years, both for normal [21] and reversed micelles [22].

Analytical Information

Since the alkyl chain length of a surfactant has a decisive influence on its physicochemical properties and hence on various biochemical applications [23, 24], each surfactant has been analyzed with respect to the uniformity of the alkyl chain by various techniques such as TLC, HPLC, GC (after hydrolysis). Some traditional surfactants, such as the Triton-series, are mixtures of a variety homologues and must correspond to the standard mixture. Surfactants consisting of only one species are characterized by a minimum purity assay which refers to the chain homologue purity. Each surfactant is further checked for appearance, solubility, identity (by FT-IR and/or NMR) and relevant trace impurities such as the respective starting material, peroxides, UV-absorbing foreign materials and metal traces (by ICP-AES). The stereochemical purity is checked by measuring optical rotation.

Applications

The surfactants (see [25] for the difference between detergent and surfactant) are of widespread importance in the detergent industry, in emulsification, lubrication, catalysis, tertiary oil recovery, and in drug delivery.

In biochemistry, the practical as well as theoretical importance of surfactants may be illustrated with the following examples: Surfactants have allowed the investigation of molecular properties of membrane proteins and lipoproteins, acting as solubilizing agents and as probes for hydrophobic binding sites [26]. The properties of surfactants, as well as further facts relevant to the particular experiments, must be carefully considered [27-29]. Surfactants have successfully contributed to the purification of receptors in their active forms [30], such as the neuropeptide receptors [31] and opiate receptors [32]. All holoreceptor- complex and reaction- center isolations require the use of a surfactant in order to separate the integral protein systems from the rest of the membrane [33].

Surfactants have been used in the investigation of the denaturation of bacteriorhodopsin [34] and in thermal stability experiments of rhodopsin [35].

The operations of exchange [36] and removal [37] of surfactants bound to membrane proteins are crucial and have been successfully applied to a wide variety of these proteins.

The effects of surfactants on the function of membrane-bound enzymes such as cytochrome P-450 [38] and (Na+ + K+)-ATPase [39] have also been determined.

Integral membrane proteins can be separated from hydrophilic proteins and identified as such in crude surfactant extracts of membrane or cells [40].

Methods for the solubilization of low-density lipoproteins have advanced the understanding of the assembly, interconversion and molecular exchange processes with plasma lipoproteins [41].

In electrophoresis, various techniques require the use of surfactants. The popular techniques of SDS-PAGE for the identification and subunit molecular weight estimation of proteins is based on a specific type of surfactant-protein interaction [42]. 2D-PAGE uses SDS in one direction and Triton X-100 in the other. This technique has been used to identify proteins containing long hydrophobic regions [44] and relies on the different binding ability of non-ionic surfactants to water-soluble and intrinsic membrane proteins. Isoelectric focusing [45], native electrophoresis and blotting [46] are other electrophoretic techniques which may need surfactants for the solubilization or transfer of membrane proteins.

In high performance liquid chromatography, common techniques such as ion-exchange HPLC, reversed-phase HPLC and sizeexclusion-HPLC may require surfactants to solubilize membrane proteins [47 48]. Ionpair HPLC requires surfactants as reagents in order to achieve the separation conditions (ionpairing) [49, 50].

Affinity surfactants have been used as reversibly bound ligands in high performance affinity chromatography [51].

Crystallization of membrane proteins was achieved using short chain surfactants, which are believed to shield the hydrophobic intermembrane part of the molecule. Thus the polar interactions betvveen individual molecules come into play, providing the stabilizing force in crystallization [52].

Surfactants are also employed to promote the dissociation of proteins from nucleic acids on extraction from biological material.

Further applications of surfactants in biochemistry are the solubilization of enzymes in apolar solvents via reversed micelles [53] and the isolation of hydrophobic proteins [54].

In analytical chemistry, surfactants have been recognized as being very useful for improving analytical methodology, e.g. in chromatography and luminiescence spectroscopy [55, 56]. For applications requiring highest quality products, we offer a range of BioUltra standard precipitation reagents.